(a) Field of the Invention
This invention relates generally to the art of amperometric measurement and to devices of the type used for quantitative electrochemical analysis methods where the concentration of an electroactive species in a fluid medium is to be measured or monitored; more particularly, this invention relates to an improved method of quantitative electroanalytical determination of elemental hydrogen.
The invention further relates to an amperometric hydrogen sensor.
(b) Description of the Prior Art Electrochemical cells of the type used for quantitative electrochemical analysis are well known in the art and generally include a working or sensing electrode having a defined or electroanalytically effective surface, a counter electrode, an optional guard electrode, an electrolyte in contact with the electrodes and a membrane that is substantially impermeable to the electrolyte but is permeable to the electroactive species of interest and defines the sensor face in terms of a barrier between the electrolyte space, notably the electrolyte film on top of the sensing electrode, and the ambient medium that contains the electroactive species of interest.
For amperometric analytical operation, the working electrode of such a cell arrangement is polarized by a constant DC voltage to furnish a current whose steady state magnitude is proportional to the activity of the electroactive species of interest. Cells of this type and their operation and use for determination purposes are discussed in the following illustrative U.S. Pat. Nos. 2,913,386, 3,071,530, 3,223,608, 3,227,643, 3,325,378, 3,372,103, 3,406,109, 3,429,796, 3,515,658, 3,622,488 and 4,096,047 as well as in British Published Application No. 1,013,895.
The above mentioned U.S. Pat. No. 2,913,386 to Leland E. Clarke considered as the pioneering patent already teaches that methods of this type are suitable for use in determining either electro-reducible or electro-oxidizable gases using electroanalytical devices also called "membrane-covered polarographic detectors". As the term "polarographic" has also been used for techniques based on the dropping mercury electrode and operating either in a voltametric or galvanic mode, the term "membrane-enclosed amperometric cell" or MEAC is used herein to refer to electroanalytical probes such as the "Clark Cell" and modifications thereof including those that use guard electrodes and various devices to improve operation, reliability, sensitivity and maintenance.
However, while the determination of electro-reducible gases, notably of oxygen, by means of MEAC sensors is widely accepted (cf. for example Hitchman, M. L., Measurement of Dissolved Oxygen, ISBN 0-471-03885-7), previous attempts to use this technique for determination of electro-oxidizable gases, notably hydrogen, have not met with comparable success.
To the best of applicant's knowledge, measurement of hydrogen with a MEAC type sensor was first mentioned by Sawyer et al, Anal. Chem. 31, 2 (1959) who reported that their attempt to detect hydrogen by this principle was totally unsuccessful. Then, Greene, M. W. et al, have disclosed a hydrogen sensing method by means of a modified MEAC in the above mentioned U.S. Pat. No. 3,325,378, and that patent cites various fields of technology where hydrogen sensing is important, e.g. detection of hydrogen leakage whenever hydrogen is used as a reactant, coolant or fuel, notably for prevention of forming explosive mixtures of hydrogen and air.
While the need for sensing and quantitative determination of hydrogen has by no means diminished, the electroanalytical method using MEAC type sensors has not been found to be generally suitable and hydrogen sensors have not been made available commercially so far.
The feature common to prior art hydrogen sensing with a MEAC according to the Greene et al patent and according to various other reports (von Gruniger et al, Helv. Chim. Acta 61 (1978) 2375; Srinivasan et al, Anal. Chem. 53 (1981) 928; Mills et al, Anal. Chem. 53 (1981) 1254; Niedrach et al, Anal. Chem. 54 (1982) 1651) is that stability of measurements and notably the absence of detrimental drift generally required some sort of surface enlargement at the anodic sensing surface made of a platinum metal, e.g. by using etching techniques or by using micro-crystalline layers of the type obtained by electrolytic platinization or deposition techniques including layers of platinum-black.
As disclosed in the Greene et al Patent, platinum-black is preferred over an etched platinum surface as the anodic sensing surface because it produces a more stable output, and it is the consistent teaching of the art that surface increase or "roughening" leading to enhancement and activation of the platinum sensing area is required at the anodic electrode of MEAC type hydrogen sensors thus precluding the use of conventionally polished, i.e. substantially smooth platinum metal anodes for that purpose.
The use of polished smooth electrode surfaces of noble metals including platinum is quite conventional in MEAC type sensors for oxygen measurement and the advantages of using a smoothly polished platinum surface over platinum-black or other area-enhanced platinum surfaces at the sensing electrode is apparent to everybody familiar with the electroanalytical art: reproducibility and definition of the sensing area as well as ease of manufacture, maintenance, and maintenance control are, of course, far better when using smoothly polished platinum surfaces and nobody in the electroanalytical art would willingly give up these advantages unless this was believed to be absolutely unavoidable.
From this it will be apparent that prior art hydrogen sensing methods with MEAC sensors were assumed to be inoperative with smooth anodic sensing surfaces of platinum metal if an output of acceptable stability was to be obtained.